Diesel and lean-burn gasoline engines generally operate under highly oxidizing conditions (i.e., using much more than is necessary to burn the fuel), typically at air/fuel ratios greater than 17, and usually about 20 to 23. Under these highly lean conditions, typical three-way catalysts (for converting hydrocarbons, NO.sub.x and CO containing primarily noble metals such as Pt, Pd, and Rh supported on .UPSILON.-alumina exhibit little activity toward NO.sub.x reduction, as their reduction activity is suppressed by the presence of excess oxygen. Recently, copper-ion exchanged zeolite catalysts, especially Cu-ZSM-5 catalysts among others, have been shown to be active for selective reduction of NO.sub.x by hydrocarbons in the presence of excess oxygen. Platinum-ion exchanged zeolite catalyst, such as Pt-ZSM-5 is also known to be active for NO.sub.x reduction by hydrocarbons under lean conditions. However, this catalytic activity is significant only in a narrow temperature range around the lightoff temperature of hydrocarbon oxidation. All the known lean-NO.sub.x catalysts reported in the literature tend to lose their catalytic activity for NO.sub.x reduction when the catalyst temperature reaches well above the lightoff temperature of hydrocarbon oxidation. This narrow temperature window of the lean-NO.sub.x catalysts is considered to be one of the major technical obstacles, because it makes practical application of these catalysts difficult for lean-burn gasoline or diesel engines.
The present invention overcomes many of the shortcomings of the prior art.